layout: Fix missing switch to MTK

This bit was missed, probably because it uses the cairo type
instead of Meta.Rectangle.

Fixes 80237b1082
Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6970

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2930>

js/ui/layout.js

@@ -1461,7 +1461,7 @@ class ScreenTransition extends Clutter.Actor {
 
         Main.uiGroup.set_child_above_sibling(this, null);
 
-        const rect = new imports.gi.cairo.RectangleInt({
+        const rect = new Mtk.Rectangle({
             x: 0,
             y: 0,
             width: global.screen_width,